1. Field of the Invention
This invention relates generally to boilers and sootblowers and, in particular, to methods and apparatus for removing ash deposits on heat exchangers of the boilers and for minimizing a flowrate of steam or other cleaning fluid through the sootblowers when not actively cleaning the ash deposit.
2. Description of Related Art
In the paper-making process, chemical pulping yields, as a by-product, black liquor which contains almost all of the inorganic cooking chemicals along with the lignin and other organic matter separated from the wood during pulping in a digester. The black liquor is burned in a boiler. The two main functions of the boiler are to recover the inorganic cooking chemicals used in the pulping process and to make use of the chemical energy in the organic portion of the black liquor to generate steam for a paper mill. As used herein, the term boiler includes a top supported boiler that, as described below, burns a fuel which fouls heat transfer surfaces.
A Kraft boiler includes superheaters in an upper furnace that extract heat by radiation and convection from the furnace gases. Saturated steam enters the superheater section and superheated steam exits at a controlled temperature. The superheaters are constructed of an array of platens that are constructed of tubes for conducting and transferring heat. Superheater heat transfer surfaces are continually being fouled by ash that is being carried out of the furnace chamber. The amount of black liquor that can be burned in a Kraft boiler is often limited by the rate and extent of fouling on the surfaces of the superheater. The fouling, including ash deposited on the superheater surfaces, reduces the heat absorbed from the liquor combustion, resulting in reduced exit steam temperatures from the superheaters and high gas temperatures entering the boiler bank.
Boiler shutdown for cleaning is required when either the exit steam temperature is too low for use in downstream equipment or the temperature entering the boiler bank exceeds the melting temperature of the deposits, resulting in gas side pluggage of the boiler bank. In addition, eventually fouling causes plugging and, in order to remove the plugging, the burning process in the boiler has to be stopped. Kraft boilers are particularly prone to the problem of superheater fouling. Three conventional methods of removing ash deposits from the superheaters in Kraft boilers include:
1) sootblowing, 2) chill-and-blow, and 3) waterwashing. This application addresses only the first of these methods, sootblowing.
Sootblowing is a process that includes blowing deposited ashes off the superheater (or other heat transfer surface that is plagued with ash deposits, with a blast of steam from nozzles of a lance of a sootblower. A sootblower lance has a lance tube for conducting the steam to a nozzle at a distal end of the lance. Sootblowing is performed essentially continuously during normal boiler operation, with different sootblowers turned on at different times. Sootblowing is usually carried out using steam. The steam consumption of an individual sootblower is typically 4-5 kg/s; as many as 4 sootblowers are used simultaneously. Typical sootblower usage is about 3-7% of the steam production of the entire boiler. The sootblowing procedure thus consumes a large amount of thermal energy produced by the boiler.
The sootblowing process may be part of a procedure known as sequence sootblowing, wherein sootblowers operate at determined intervals in an order determined by a certain predetermined list. The sootblowing procedure runs at its own pace according to the list, irrespective of whether sootblowing is needed or not. Often, this leads to plugging that cannot necessarily be prevented even if the sootblowing procedure consumes a high amount of steam. Each sootblowing operation reduces a portion of the nearby ash deposit but the ash deposit nevertheless continues to build up over time. As the deposit grows, sootblowing becomes gradually less effective and results in impairment of the heat transfer. When the ash deposit reaches a certain threshold where boiler efficiency is significantly reduced and sootblowing is insufficiently effective, deposits may need to be removed by another cleaning process.
A steam sootblower, typically, includes a lance having an elongated tube with a nozzle at a distal end of the tube and the nozzle has one or more radial openings. The tube is coupled to a source of pressurized steam. The sootblowers are further structured to be inserted and extracted into the furnace or moved between a first position located outside of the furnace, to a second location within the furnace. As the sootblowers move between the first and second positions, the sootblower rotates and adjacent to the heat transfer surfaces. Sootblowers are arranged to move generally perpendicular to the heat transfer surfaces.
Some of the platens having heat transfer surfaces have passages therethrough to allow movement perpendicular to the heat transfer surfaces. The movement into the furnace, which is typically the movement between the first and second positions, may be identified as a “first stroke” or insertion, and the movement out of the furnace, which is typically the movement between the second position and the first position, may be identified as the “second stroke” or extraction. Generally, sootblowing methods use the full motion of the sootblower between the first position and the second position; however, a partial motion may also be considered a first or second stroke.
As the sootblower moves adjacent to the heat transfer surfaces, the steam is expelled through the openings in the nozzle. The steam contacts the ash deposits on the heat transfer surfaces and dislodges a quantity of ash, some ash, however, remains. As used herein, the term “removed ash” shall refer to the ash deposit that is removed by the sootblowing procedure and “residual ash” shall refer to the ash that remains on a heat transfer surface after the sootblowing procedure. The steam is usually applied during both the first and second strokes.
Rather than simply running the sootblowers on a schedule, it may be desirable to actuate the sootblowers when the ash buildup reaches a predetermined level. One method of determining the amount of buildup of ash on the heat transfer surfaces within the furnace is to measure the weight of the heat transfer surfaces and associated superheater components. One method of determining the weight of the deposits is disclosed in U.S. Pat. No. 6,323,442 and another method is disclosed in U.S. patent application Ser. No. 10/950,707, filed Sep. 27, 2004, both of which are incorporated herein by reference. It is further desirable to conserve energy by having the sootblowers use a minimum amount of steam when cleaning the heat transfer surfaces.